add spidev file path in variant.h

set file path of spi
1. /dev/spidev1.1 (default SSP2)
2. /dev/spidev32766.1 (SSP3)
NOTE: Use only one device at a time. Comment the defination of LINUX_SPIDEV - one that you're no using
add author name of SPI library

add author name in SPI.c and SPI.h
Add default SPI clock frequency

Default clock frequency for arduino is 4MHz
Update SPIClass and SPI_CLOCK_DIV definition

change value of SPI_CLOCK_DIV4 to 0x00 (case 0)
remove ret file descriptor as it is redundant variable in code
remove device pointer - it is defined in variant.h
remove spi_ioc_transfer structure - will define in SPI.c file
Update SPI:setBitOrder function

No ioctl function call to set bit order in previous version of function
Remove default SPI parameters from SPI constructor

Default parameters like SPI speed and File path to spidev is declared in variant.h
Using those values instead declaring it again in SPI.c file
Update functionality of SPIClass:transfer()

No method to provide full duplex transfer in previous version
Add ioctl() function and spi_ioc_transfer structure to achieve full duplex communication
Update setDataMode and setClockDivider functions of SPI
Add default values to set in SPIClass:begin()

Default values of Data Mode (Mode0), ClockDivider(SPI_CLOCK_DIV4), BitOrder(MSBFIRST) are set